Geophysical analysis of marine gas hydrate structures

• Main Author: Attias, Eric
• Other Authors: Minshull, Timothy
• Published: University of Southampton 2017
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.729776

Gas hydrate deposits are known to store vast amounts of methane, and occur worldwide in marine and permafrost regions. Methane emissions driven by hydrate dissociation may contribute to submarine slope failures, geohazards to deep water infrastructures, and possibly climate change. Alternatively, hydrates are perceived as a viable energy resource. These environmental and economic implications mean that gas hydrate research is of both academic and industrial interest. To determine the environmental impact or economic potential of gas hydrate accumulations in any given geologic setting with a high level of confidence, it is mandatory to acquire lithological and geophysical information for a well-constrained joint interpretation. Robust delineation and quantification of gas hydrate structures is not a trivial task, due to inherent uncertainties from the absence of information regarding the physical properties of the reservoir of interest. In this thesis, I develop a rigorous joint interpretation scheme using marine controlled-source electromagnetic (CSEM), seismic and core data coupled by effective medium modelling, for the detection, delineation, and quantification of marine gas hydrate structures. The study area for this research is the CNE03 pockmark, situated on the Norwegian continental slope, Nyegga region, offshore Norway. The CNE03 pockmark is underlain by a pipe-like structure, where gas hydrate and free gas coexist. Marine CSEM data and sediment cores were acquired from the CNE03 pockmark, integrated and interpreted with collocated high-resolution two-dimensional seismic reflection and three-dimensional tomographic seismic data. The CNE03 pipe-like hydrate structure is detected and characterised using unconstrained and seismically constrained CSEM inversions of data obtained by ocean bottom electric field receivers (OBE). The unconstrained CSEM inversions detected the CNE03 pipe-like structure satisfactorily though with undefined and diffusive margins, which is mitigated by the seismically constrained inversions that improved the delineation of the CNE03 boundaries significantly. High-resolution resistivity imaging of the CNE03 pipe-like structure is achieved by a combined CSEM inversion of both the OBE and 3-axis towed electric field receiver (Vulcan) data. Robust quantification of hydrate content within the CNE03 structure is derived by comparison between CSEM and seismic datasets with joint elastic-electrical effective medium modelling scheme. The work I present in this thesis provides an integrated approach to elucidate both structural and fluid properties of sub-seafloor gas hydrate and free gas deposits. The joint interpretation framework applied here could also be utilised to map and monitor seafloor mineralisation, freshwater reservoirs, carbon capture and storage sites, and near-surface geothermal systems.